This invention relates generally to the treatment of selected tissue by inter-vivo radiation, for example to prevent restenosis in a patient's vascular system, and, more particularly, to such treatment using a radioactive-tipped source wire.
Coronary vessel stenosis is commonly treated using percutaneous transluminal coronary angioplasty (PTCA), or balloon angioplasty. Several hundred thousand such procedures are performed annually in the United States, and this is believed to constitute about half of the worldwide number of such procedures. The PTCA procedure is popular, because of its relatively high success rate and its minimal invasiveness as compared with coronary bypass surgery.
However, patients treated by PTCA suffer from a high incidence of restenosis, brought on by injury to the arterial wall during the PTCA procedure. In some patients, the injury initiates a repair response in the form hyperplastic growth of the vascular smooth muscle cells in the traumatized region. This hyperplasia narrows the lumen that was opened by the PTCA procedure and necessitates a repeat PTCA or other procedure, e.g., bypass surgery, with attendant high cost and added patient risk.
Intravascular radiotherapy (IRT) has shown significant promise in the prevention or long-term control of restenosis following a PTCA procedure. It also has shown promise in the prevention or long-term control of stenosis following a cardiovascular graft procedure or other trauma to the vessel wall. Typically, IRT is performed by advancing a flexible, radioguide catheter through the patient's cardiovascular system until the catheter's distal tip is located at or near the vessel region to be treated, e.g., the region previously subjected to the angioplasty procedure. A remote afterloader then advances a treatment catheter in the form of a wire having a radiation source at its tip, i.e., a source wire, through the radioguide catheter until the radiation source reaches the vessel region to be treated. The radiation source is held in this region for a prescribed time duration, calculated to deliver an effective dose of radiation to the vessel region to be treated, after which the source wire is withdrawn back into the afterloader for appropriate shielding.
The source wire typically takes the form of a solid lead formed of a nickel-titanium alloy, having the requisite levels of flexibility, springiness, lubricity, mechanical strength, and shape memory retention. The radiation source typically takes the form of a radioactive isotope such as iridium, embedded within the source wire, at its distal tip.
The radioactive source at the end of the source wire must be handled with extreme care. Even short exposures at close distances can result in radiation injury. It is therefore extremely important that the afterloader, which controls the advancement and retraction of the source wire within the radioguide catheter, be manufactured for high reliability, and that it be configured to controllably position the source wire within the radioguide catheter with extreme accuracy and precision. In the past, the controllers that position the distal tip of the source wire within the radioguide catheter have included special optical or mechanical sensors for sensing when that distal tip is located at a home or reference position within the afterloader.
Although such optical and mechanical sensors have operated generally satisfactorily in detecting the presence of the source wire's distal tip at its reference position within the afterloader, the sensors' performance can degrade over time. This degradation is due, in part, to debris accumulating at the site of the sensor. One source of such debris is the radioguide catheter, itself. As the source wire is cycled into and out of a catheter lumen, a certain amount of catheter material is scraped away, and this material is drawn into the afterloader's drive mechanism. This debris can obscure the view of an optical sensor that senses the presence of the source wire's distal tip, and it can accelerate the wear of a mechanical sensor that senses such presence. As a consequence, the presence of the source wire's distal tip at its reference position cannot necessarily be known with the desired level of accuracy and precision.
It should, therefore, be appreciated that there is a need for an improved intravascular radiotherapy apparatus, and related method, that senses the presence of the source wire's distal tip at its home or reference position within the afterloader, while avoiding performance degradation caused by an accumulation of debris from such sources as the radioguide catheter. The present invention satisfies this need.